One of the methods for optimizing the investment in optical fiber communications, which corresponds to an increasing demand for information transmission by optical fiber communications, is to increase the spectral efficiency by adopting a modulation system more efficient for information to be transmitted. In order to increase the spectral efficiency, modulation systems based on quadrature phase shift keying (QPSK) have been developed in higher-capacity optical communication systems. In the QPSK system, information is encoded into four kinds of phase levels. Thus two-bit binary signals can be encoded per one symbol to be transmitted. Whereas the conventional on-off keying (OOK) has achieved information transmission with one bit per one sample, the QPSK modulation technology enables double amount of information to be transmitted with the same necessary bandwidth of optical spectrum.
Quadrature amplitude modulation (QAM) technology is one of the technologies that makes it possible to increase the communication capacity further by improving the spectral efficiency of one channel. In the QAM system, a symbol is encoded into a phase level and an amplitude level and configured as a combination of multilevel modulations in quadrature phases. In the 16QAM system, information is converted into 16 levels, that is, four-bit binary codes per symbol, for example. This makes it possible to increase the optical spectral efficiency twice compared to the QPSK system. The QAM system is an effective method for increasing the communication line capacity.
The QAM system can be implemented by using an optical IQ modulator. In the IQ modulator, two independent Mach-Zehnder devices are driven by electric signals. These are called child Mach-Zehnder modulators (MZMs). The child MZMs modulate the phase and intensity of the same optical carrier. The optical phase of one output of the two child MZMs is relatively delayed by 90 degrees before recoupling. The phase delay between the outputs of the child MZMs, which is called a quadrature phase angle, is ideally 90 degrees in a 180-degree method. The IQ modulator is used in the QAM modulation system and QPSK modulation system. The IQ modulator provides an efficient and demonstrated method to implement the QAM modulation.
It has been known, however, that in the IQ modulator, there is a drift of a direct current (DC) bias due to a temperature change or aged deterioration of a device. Three kinds of biases are affected by the drift. Specifically, these biases are DC biases for the two child MZMs, and a DC bias used for setting the angle between the output optical signals of the child MZMs at the quadrature phase angle. This has been already known with respect to the QPSK system, and it has been known that there is a drift with respect to the QAM system if a modulator with the same structure is used. If the drift occurs in the DC bias, the modulator is made to operate inaccurately, which causes transmission signal degradation. As a result, the signal quality in a receiving part deteriorates, or in worst case, it becomes impossible to decode received signals. With respect to the OOK, phase shift keying (PSK) modulation, and QPSK, the bias of the modulator is controlled by using an automatic bias control (ABC) circuit that compensates for a variation in the DC bias. This enables the above-described problem to be solved.
An example of a method of controlling such an optical modulator by the QPSK modulation system is described in Patent Literature 1. In the QPSK modulation system, the phase modulation of an I-arm and a Q-arm of the optical modulator is performed with an electric signal having an amplitude of 2×Vπ that is configured by using a peak, a valley, and a peak in optical intensity characteristics depending on driving voltages of the optical modulator. Here, Vπ represents a voltage by which the phase of the optical modulator varies by π, and is also called a half-wavelength voltage. Patent literature 1 discloses a method of controlling a bias voltage of an optical modulator by superimposing a low-frequency signal with a frequency f0 (hereinafter also referred to as a dither signal) on the amplitude of a driving signal.
On the other hand, in recent years, Nyquist modulation has been performed in the QPSK system in optical communications. The Nyquist modulation is to perform optical modulation after limiting band of an electric driving waveform of an optical modulator through a low-pass filter that makes only a Nyquist frequency pass. The Nyquist frequency is a frequency with half the baud-rate of a modulation signal. In a low-pass filter used practically, its roll-off coefficient α is set at approximately 0.1. Because of the band limitation, the optically modulated optical spectrum can be narrowed to half, compared to the optical spectrum before applying the low-pass filter. As a result, the wavelength interval can be decreased in wavelength division multiplexing (WDM) transmission; accordingly, it becomes possible to extend the capacity of a WDM transmission device. Although the Nyquist modulation has advantages in the WDM transmission as described above, the driving signals are not conventional two kinds of binary digital signals but electric signals obtained by the synthesis of analog waveforms. Such analog electric signals are included in signals based on a 16QAM system or an orthogonal frequency division multiplexing (OFDM) system. If the driving signal is such an analog signal, it is impossible to perform the bias control of the optical modulator by means of the method described in Patent Literature 1.
Patent Literature 2 discloses an example of an optical transmitter in which the bias control of an optical modulator can be performed even though the above-described analog driving signal is used. The optical transmitter described in Patent Literature 2 is configured to select a polarity for an error signal properly depending on an average modulation degree of an analog driving signal by including an error-signal polarity selection unit. Specifically, if the average modulation degree is greater than 50%, the error-signal polarity selection unit selects non-inversion of polarity. In contrast, if the average modulation degree is less than 50%, the error-signal polarity selection unit selects inversion of polarity. A bias control unit controls the bias voltage of the optical modulator based on an error signal with the polarity that the error-signal polarity selection unit has selected.
However, the optical transmitter described in Patent Literature 2 has the problem that the bias control cannot be performed if the average modulation degree is equal to 50%, because the average optical output level does not vary even if the bias voltage varies. Patent Literature 3 discloses a technique for solving this problem.
FIG. 14 illustrates a configuration of a related optical transmitter 11 for QPSK described in Patent Literature 3. The related optical transmitter 11 includes a QPSK modulator 8, a monitor unit 9A, and a bias control unit 93A. Here, the monitor unit 9A includes a photodiode (PD) 87, an average optical intensity monitor 91, an optical intensity AC component monitor 92, an I/V conversion unit 94, an f0 generation unit 95, band-pass filters (BPF) 96A and 96B, and a synchronous detection unit 97.
The photodiode (PD) 87 receives part of output light from the QPSK modulator 8. The I/V conversion unit 94 generates a monitor signal obtained by converting current output from the PD 87 into voltage. The monitor signal is inputted into the average optical intensity monitor 91 and the optical intensity AC component monitor 92, respectively. The average optical intensity monitor 91 detects an average value of the monitor signal. The optical intensity AC component monitor 92 detects an AC component included in the monitor signal. Here, an RMS (root mean square value: AC effective value)-DC converter can be used as the optical intensity AC component monitor 92.
The synchronous detection unit 97 synchronously detects a low-frequency signal inputted from the f0 generation unit 95 and f0 components inputted from the BPF 96A and BPF 96B. The bias control unit 93A is configured to perform the bias control of the QPSK modulator 8 selectively using one of the f0 components indicative of the detection results of the monitors 91 and 92 that have been extracted by the synchronous detection in the synchronous detection unit 97.
That is to say, if a driving amplitude Vd is in a state of crossing Vπ, the bias control unit 93A performs the bias control using the result obtained by synchronously detecting the detection results of the optical intensity AC component monitor 92 in the synchronous detection unit 97. Specifically, the bias control unit 93A controls a bias voltage so that the optical intensity AC component will be at a minimum, for example. In contrast, in cases other than the above, the bias control unit 93A performs the bias control by using the result obtained by synchronously detecting the detection result of the average optical intensity monitor 91 in the synchronous detection unit 97.
As described above, the related optical transmitter 11 is configured to use the average optical intensity monitor 91 and optical intensity AC component monitor 92 in a mutually complementary manner; consequently, it becomes possible to perform the bias control without depending on the driving amplitude.